The use of electrophotographic copiers as computer output printing devices is well known. In one such system, with which the present invention is related, a liquid electrostatic copier includes a rotatable drum which has a photoconductive surface. The surface of the drum is first moved past a charging station. An image to be copied is then projected onto the drum at an exposure station. After leaving the exposure station, the drum has a latent electrostatic image thereon. The drum then moves through apparatus which includes a liquid developer having charged toner particles suspended in a suitable carrier liquid or dispersant. The drum then leaves the liquid developer with a developed electrostatic image produced by the toner particles being attracted to the latent image on the drum. After the drum leaves the liquid developer, it is brought in contact with paper. The toner is transferred from the surface of the drum onto the paper. Electrostatic liquid developer systems of the type mentioned have been described in numerous patents and publications.
It is advantageous to remove excess liquid developer from the drum after the image has been developed. Removing the excess liquid developer from the drum reduces the amount of carrier liquid that is transferred to the paper and minimizes the likelihood of smudging of the resultant image. Furthermore, when the excess liquid developer is removed from the drum less heat is required to fix the image transferred from the drum to the paper. Additionally, as a result of reducing the amount of liquid developer transferred to the paper the quantity of vapors generated by evaporation of the carrier liquid to pollute the air is also reduced and a more precise and controllable transfer of the dry toner particles to the paper is accomplished. Removing and reusing excess dispersant is an important feature because petroleum products are used in the manufacture of dispersant and the cost of such products have been increasing dramatically.
In the prior art, absorbent rollers and driers have been used for removing the excess dispersant. It is also known to use air knives for this purpose. Such air knives utilize an air jet to blow the dispersant material off the drum while leaving the solid toner particles electrostatically retained on the electrostatic surface thereof. Air knives of this type have been disclosed in U.S. Pat. Nos. 3,741,643; 3,100,426; 3,811,765 and others. The present invention relates to such an air knife in combination with other means and methods to return the dispersant to a reservoir.
One of the considerations in using an air knife to remove excess dispersant from a drum is to provide a reliable and efficient arrangement without the use of excess power. For example, if the air jet employed is too wide, then more power is required to produce the jet and the tendency of the developed image to become distorted is increased. On the other hand, if the jet is too narrow, then the excess carrier liquid will not be removed sufficiently.
In designing an air knife, it is important that the air jet projected be uniformly distributed across a relatively wide area of the drum. If this is not done, a distortion of the image may result.
Also, it is important that the air knife alone not be relied upon to return the removed excess dispersant to a reservoir. In addition, it is desirable to remove some of the excess dispersant from the drum by metering or otherwise prior to subjecting it to the air jet from the air knife.